Coincidentally, I also received a totally unexpected call from police earlier this week. I wasn't suspected of committing a crime. Apparently helping them get in touch with my flatmate - who unbeknownst to me had
reported a crime - didn't incriminate me or my flatmate. Don't get me wrong, I'm sure a
lawyer would rather I had that two minute conversation in their paid presence, but I'm not really sure where they'd have helped me avoid accidentally intimating that I might be a criminal: I think I'd have done that more effectively by automatically assuming I was being investigated.
Assuming instantly pleading the Fifth helps misses the bit where police and especially more powerful organizations like the FBI jump to and act on a lot of conclusions that aren't admissible in court (and of course have plenty of capability to find non-verbal, admissible evidence of wrongdoing if motivated to look for it, even if you're innocent). If they're casually following up a dubious lead, which they probably are if you're not expecting their visit and don't have pot plants or a server mirroring Wikileaks in your conservatory, you probably don't want to fit their mental model of a suspect, which probably means you being at least slightly curious about why they want to speak to you, and at least willing to entertain the possibility that you're not the suspect. Statistically, you're more likely to plead the Fifth if you're guilty.
(This probably doesn't apply when they're picking on you because you're a genuine suspect or there are political reasons for them trying to find out about your friends or organization.)